Although it was first observed over thirty years ago that carcinogenic substances are mutagenic substances, only recently has a test been developed which is capable of proving this hypothesis. The Ames test, developed by Dr. Bruce Ames, has found that many chemical carcinogens or cancer-causing agents are potent frameshift mutagens or substances which may be metabolized in cells to forms that are frameshift mutagen. In brief, the Ames test is an assay for screening compounds for their ability to revert a series of known frameshift mutations in the hisD gene to wild type, which is readily assayed because wild-type mutated cells form colonies on medium that lack histidine. The rational of the test is based on the premise that mutagenesis and cancer induction both result from alteration of the DNA of a cell. Experimentally, over 90% of the carcinogens tested have been determined to be mutagens. The Ames test, as indicated above, measures the rate of reversion of histidine auxotrophs of Salmonella to prototrophy in both the presence and absence of the chemical being tested. If the chemical is mutagenic, it will increase the reversion rate. The test also gives some idea of how powerful a mutagen, or how potentially hazardous, a chemical is by a number of revertants that arise. The more powerful the mutagen, the greater number of revertants.